There are numerous instances where operating conditions experienced by an article, or a component of a machine, place differing materials property requirements on different portions of the article or component. Examples include a crankshaft in an internal combustion engine, a piston rod in a hydraulic cylinder, planetary gears for an automobile transmission or the metal head of a carpenter's claw hammer. In a crankshaft, the journals must have hardened surfaces to resist wear during operation, but the crankshaft must also be tough enough to withstand transients in loading. Similarly, a piston rod must have a hard surface to avoid nicks, which might otherwise cause leaks of hydraulic fluid, but toughness to withstand transients in loading is also needed. In these two examples, the requirements may be met by fabricating the parts from nodular iron, or a medium carbon steel, and then induction hardening the articles to obtain the hard surface layer in the desired portions of the articles. The depth of the hardened layer produced by induction hardening is frequently between about 0.03 inch to 0.10 inch. In each of these articles, the surface of the article is differentially austenized, typically within a fraction of a minute, and then quenched to develop a hard martensite surface, which then may be tempered as desired.
A planetary gear for an automobile transmission is typically made from a low carbon steel, masked, then carburized. A carburized surface layer, limited to unmasked portions of the surface and generally less than about 0.04 inch in depth, contains sufficient carbon that it becomes substantially harder than the core of the gear during subsequent heat treatment. The hard carburized layer provides wear resistance in the gear teeth, while retaining toughness in the interior of the gear. Although carburizing is sometimes an alternative to induction or flame hardening, it should be regarded as selective surface alloying, rather than differential heat treatment.
A hammer head must be able to withstand pounding against nail heads, but the claws must have sufficient toughness to withstand extracting nails from wood. In this example, the entire striking end of the steel hammer head is austenized, in a minute or two, and then the head is quenched and tempered. This example differs from the crankshaft in that the entire striking end of the hammer is differentially heat treated, rather than just a thin surface layer.
One common feature of the well-known differential heat treatment processes employed in these examples is that each is applied to iron-carbon alloys, where carbon is the atomic species essential to hardening. Because carbon atoms diffuse so rapidly in iron-carbon alloys, each differential heating process can be performed within a few minutes. There is sufficient latitude in austenizing that it is generally not necessary to accurately control the temperature distribution within the differentially heated portions of the articles. Thus, it is generally not necessary to make any provision in the process for keeping the portions of the articles not being heat treated cool.
A turbine disk for a gas turbine engine is an example of another type of article where different properties in various portions of the article are preferred. Such disks are typically made from nickel-base superalloys, because of the temperatures and stresses involved in the gas turbine cycle. In the hub portion where the operating temperature is somewhat lower, the limiting material properties are often tensile strength and low-cycle fatigue resistance. In the rim portion where the operating temperature is higher because of proximity to the combustion gases, resistance to creep and hold time fatigue crack growth (HTFCG) are often the limiting material properties. HTFCG is the propensity in a material for a crack to grow under cyclic loading conditions where the peak tensile strain is maintained at a constant value for an extended period of time. By contrast, in conventional low-cycle fatigue testing the peak tensile strain is reached only momentarily before reduction in the strain begins.
It has not heretofore been possible to conveniently and reliably heat treat a disk to obtain such a combination of different properties in the different regions of a disk. As a consequence, most turbine disks have been heat treated with a process designed to provide a compromise set of properties throughout the entire disk. The various conditions which, taken together, have created such a formidable problem for heat treating, include the following. The disk itself, particularly for a large aircraft gas turbine engine, is generally about 25 inches in diameter. The rim portion of a disk, which must have the same properties throughout its extent, is an annular ring whose dimension in both axial and radial directions is greater than about 2 inches. These dimensions indicate that a large volume of metal must be heated. The nickel-base superalloys must be heated to temperatures above about 2000.degree. F., for times of two hours or longer, to achieve the structure which provides the improved creep and HTFCG resistance needed for this application. The hub portion of the disk, however, must be kept below about 1900.degree. F. to avoid altering its structure and properties.
The preceding combination of problems has been so formidable that other approaches to developing turbine disks having different properties in their hub and rim portions have been developed. One such approach, which provides a dual alloy disk by forge enhanced bonding of two different alloys for the rim and hub portions of the disk, is described in U.S. Pat. No. 5,100,050, assigned to the assignee hereof, which is incorporated herein by reference. It is noted that while the present invention was developed to provide differential heat treatment, and the resulting differences in properties between different portions of an article, in an article comprised of a single alloy, it may also be advantageously employed in heat treating a dual alloy disk made by the referenced process, or by any other appropriate process, in which the rim and bore or hub must be heat treated at different temperatures to achieve optimum properties in each.
The present invention fulfills the need for a differentially heat treated article, and an effective apparatus and process for providing such an article, and provides related advantages.